This invention relates to plasma glow discharges and, in particular, to glow discharges sustained by microwave radiation.
The use of plasma glow discharges for processing semiconductor wafers, either for etching or deposition, has greatly increased in the last several years. However, several broad trends have caused a need for a new method for generating the plasma. In the past, the plasma has typically been generated by coupling relatively high power (five hundred watts or more) of RF energy into a chamber containing one or more gases. The coupling is by way of parallel electrodes forming a capacitor, one electrode of which is adapted to hold the semiconductor wafer.
As wafer diameters have increased, the amount of power has increased to maintain a given power density. As the geometry of devices has decreased in size, the devices become ever more delicate. The point has been reached, or will very soon be reached, where the devices simply cannot withstand the applied power passing through the wafer.
An early solution to this problem has been the use of screen grids within the chamber to isolate the wafer being processed. Another solution is the so-called downstream etcher, a name covering several types of devices wherein one hopes that the lifetime of the reactive, non-charged species exceeds the time it takes for the species to drift to the surface of the wafer. One type of downstream device uses microwaves to sustain the discharge.
With microwaves, or any other type of discharge, there are two modes of operation. The first is the starting mode. The second is the sustained running mode. Just as with an automobile engine or a fluorescent lamp or any of a host of other common devices, the two modes are very different. Just as with these other devices, the starting mode is the hardest on the equipment. There is relatively little said in the prior art on how to initiate a plasma discharge in a reliable, repeatable, commercially feasible manner.
The chamber in which the discharge takes place has an electrical impedance. Considering the case of parallel plate electrodes, it should seem intuitively true that the impedance when there is no discharge is very different from the impedance when there is a discharge. Thus, one is faced with the problem of applying power to a nonresonant, reactive load. As is well known, such a load causes a great deal of power to be reflected back to the source. As is also well known, reflected power tends to destroy the final stage in the RF amplifier. The same is true whether the signal is at a microwave frequency or a lower frequency.
One solution to this problem has been to use relatively low Q tuned circuits in the final stages of the amplifier. This provides a broadly tuned circuit but tends to consume a lot of power.
Another solution has been to use an automatic tuning mechanism of some sort. This works reasonably well at high frequencies, but becomes more difficult at microwave frequencies. The mechanical plungers and the like tend to require frequent adjustment and are undesirable for the customer and the equipment manufacturer.
Yet another solution, typified by U.S. Pat. No. 4,175,235 - Niwa et al., uses internal electrodes and an electric spark generator for initiating the glow discharge. Depending upon the gases utilized, the electrodes can be corroded or deposited upon or, worse, can provide contaminating ions into the reaction chamber even when the electrodes are not carrying a spark.
Another solution is to use a UV light source within the plasma chamber to initiate the glow discharge. A problem with this approach is the time it takes for the UV to provide a sufficient electron density for the glow to stabilize. During this time, the impedance of the glow varies considerably. Some manufacturers leave the UV source turned on continuously to minimize this problem. This raises problems of the reliability of the UV source and assuring that the UV is off before the unit is opened for service.
In view of the foregoing, it is therefore an object of the present invention to provide an ignitor for a microwave sustained plasma.
Another object of the present invention is to provide an improved equipment using a microwave sustained plasma for treating semiconductor wafers by enabling the microwave power source to have a fixed tune with relatively high Q components.
A further object of the present invention is to provide an ignitor located outside the discharge chamber to avoid any deterioration of the ignitor by the gases used for the plasma.